I have a Toyota Blizzard and when I drive with the front windows down the smell of diesel exhaust in the cabin is strong. It seems to be a little worse with the front windows down than with the rear window rolled up (although I have tried all combinations). This occurs while driving; the fumes are not nearly as strong while sitting and idling. With the windows closed, regardless of whether I have the AC on or not, there are no fumes. Is this normal or is it possible that there is a leak in the exhaust system?